1. Field of the Invention
The present application relates to an electronic apparatus implemented with central processing units (CPU) operated according to a micro-program rewritable externally.
2. Background Arts
Electronic apparatuses implemented with CPUs operable under a micro-program have become popular not only in the industrial application but the home application. Recently, such apparatuses perform further complicated and advanced operations and functions by rewriting the micro-program. In order to rewrite the micro-program, such apparatuses are demanded to have a function to revise a memory storing the micro-program securely under the control of the host system.
When the apparatus installs only one rewritable memory and executes the micro-program stored in this memory, the host system first sends a new micro-program in a back-up space of the rewritable memory, and then the CPU in the apparatus transfers the new micro-program into the executable space of the rewritable memory. This means that the size of the rewritable memory is inevitably to be greater than twice of the size of the new micro-program, or, the size of the new micro-program is necessary to be smaller than a half of the size of the rewritable memory.
Also, when the apparatus is implemented with a plurality of CPUs and only one interface, or bus, to connect the CPUs, all of the CPUs are inevitably implemented with a protocol common to all CPUs and a communication speed following the specification of the interface. Depending on the CPUs, and/or the functions thereof, the interface preferably provides a flexible protocol and a variable speed.